1. Field of the Invention
The present invention relates to a thin film transistor substrate, and more particularly, to a thin film transistor substrate with an invert-staggered structure.
2. Discussion of the Related Art
A thin film transistor is widely used as a switching device of a display device such as a liquid crystal display (LCD) device or an organic light emitting device (OLED).
The thin film transistor comprises a gate electrode, a semiconductor layer, a source electrode and a drain electrode. Based on an arrangement of the electrodes, the thin film transistor may be classified into a staggered structure and a coplanar structure.
In the staggered structure, the gate electrode and the source and drain electrodes are arranged in the upper and lower portions with respect to a semiconductor layer. Meanwhile, the coplanar structure is provided with the gate electrode and the source and drain electrodes arranged in the same plane. Also, the staggered structure may be classified into a normal staggered structure and an invert-staggered structure. In case of the invert-staggered structure, the gate electrode is arranged in the lower portion, and the source and drain electrodes are arranged in the upper portion. In case of the normal staggered structure, the gate electrode is arranged in the upper portion, and the source and drain electrodes are arranged in the lower portion. Generally, the staggered structure, especially the invert-staggered structure allows a decrease in the number of processes and improved interfacial properties, whereby the invert-staggered structure is generally used for the mass production.
Hereinafter, a related art thin film transistor substrate with invert-staggered structure (hereinafter, abbreviated as ‘thin film transistor substrate’) will be described with reference to the accompanying drawings.
FIG. 1 is a cross section view illustrating a related art thin film transistor substrate.
As shown in FIG. 1, the related art thin film transistor substrate comprises a substrate 10, a gate electrode 20, a gate insulating film 25, a semiconductor layer 30, a source electrode 40a, and a drain electrode 40b. 
The gate electrode 20 is patterned on the substrate 10, and the gate insulating film 25 is formed on an entire surface of the substrate 10 including the gate electrode 20.
The semiconductor layer 30 is patterned on the gate insulating film 25. The semiconductor layer 30 comprises an active layer 32 which is not doped with impurity ions, and an ohmic contact layer 34 which is doped with impurity ions. The active layer 32 is formed on the gate insulating film 25, whereby the active layer 32 functions as a charge drift channel. The ohmic contact layer 34 is formed in an interfacial region between the source electrode 40a and the drain electrode 40b, to thereby lower a charge drift barrier.
The source and drain electrodes 40a and 40b are formed on the semiconductor layer 30 while being provided at a predetermined interval from each other.
However, the related art thin film transistor substrate has a problem of low adhesive strength between the semiconductor layer 30 and the source electrode 40a, and between the semiconductor layer 30 and the drain electrode 40b. That is, because the ohmic contact layer 34 of the semiconductor layer 30 is generally formed of amorphous silicon, and the source and drain electrodes 40a and 40b are formed of metal such as copper Cu, the low adhesive strength between the semiconductor layer 30 and the source and drain electrodes 40 and 40b causes deterioration of the thin film transistor.